|Locating Mine Shafts in Butte, Montana
We were asked to locate six abandoned mine shafts on a site which was
undergoing active reclamation. Mine shafts were thought to exist after
being located on old maps, but no surface expression could be detected and the
old maps were the only evidence that a potential problem existed. It was
too dangerous and expensive to shut down the remedial effort and go looking for
them with an excavator. To make matters worse, the surface topography had
changed since the maps were made and the maps' accuracy was unknown.
Ground penetrating radar did not work well on the site because highly conductive
mine tailings covered the surface. We decided to use 3D electrical resistivity
because of the sharp contrast in conductivities between the bedrock, the
tailings, and the void space. The method was quite successful and we were
able to flag all six shaft locations prior to leaving the site. The
project was completed in two days and did not interrupt the site activity.
This is a horizontal depth slice from one of the 3D grids. The shaft cap
is shown here 2 meters below the ground surface. The shaft dips west at about a
45 degree angle, exiting the grid by the fourth depth panel.
This vertical slice through the 3D model shows the shaft going deeper and to the
west. The panels are shown starting in the east and progressing westward.